Mold grows across the ordinary indoor range, which means every temperature a house here is kept at through the year is a temperature mold can work in. There is no household thermostat setting that stops it. The number that matters is the humidity one: the EPA advises keeping indoor humidity below 60 percent relative humidity, and ideally between 30 and 50 percent.
That answer disappoints anyone who came looking for a threshold, and the reason the threshold is missing is straightforward. The public agencies that publish on mold, and the standard the remediation trade works to, both organize the subject around moisture and materials rather than around degrees. The EPA states the condition in one line: mold can grow on virtually any organic material as long as moisture and oxygen are present, and controlling indoor moisture is what controls indoor mold.
The three things mold needs, and the one you control
- A food source. Drywall paper, wood, insulation facing, dust and the backing of a carpet are all organic material. You are not going to remove them from your house.
- Oxygen. Present everywhere a person can stand.
- Moisture. This is the one variable in the list that a homeowner can move, and it is why every serious piece of guidance on the subject is really about water.
Temperature belongs to the same story, but as a passenger. It shifts how much water the air can hold, which changes where condensation forms. A cold foundation wall behind a warm humid basement is a condensation problem, and it is solved by lowering the humidity or raising the surface temperature, never by finding a number on a thermostat.
Where the humidity number comes from and how to use it
The EPA guidance is specific. Keep indoor humidity below 60 percent relative humidity, ideally between 30 and 50 percent. A cheap hygrometer will tell you which side of that line a basement sits on, and in this part of New Jersey a finished basement in August is very often on the wrong side of it without any leak at all.
Two practical readings of those numbers. First, a dehumidifier in a basement is doing prevention work, and it does it all summer, not in a crisis. Second, if a material got wet, the 24 to 48 hour window is running from the moment it got wet and not from the moment somebody noticed.
Why humid basements here are the usual answer
The pattern a technician sees most often in this service area has nothing to do with heating. It is warm humid air meeting cool masonry below grade. Along the Passaic River corridor in Passaic County and the border towns of Essex County and Bergen County, older basements carry both the cool wall and the periodic water, and finished basements add insulation and carpet that hold what condenses.
So the question that starts as a temperature question usually ends at one of four places: a leak, a drainage problem outside, condensation from air that is too humid, or material that got wet once and was never dried to a dry standard.
What the standard says, once it is already growing
The mold remediation standard, ANSI/IICRC S520, describes three conditions. Condition 1 is a normal fungal ecology. Condition 2 is settled spores and fragments. Condition 3 is actual mold growth. The job of remediation is to return an area to condition 1, and the definition of the finish line matters because it is what post remediation verification is checked against.
Two things follow for a homeowner. Nothing that only kills the surface returns an area to condition 1, and nothing works permanently while the water source is still there. The EPA puts it directly: cleaning up the mold while leaving the water problem in place means the mold will most likely come back.
What you can handle and what you should not
The EPA gives a size guide: if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases a homeowner can handle the job. Above that, and in any case where the growth followed sewage or floodwater or sits inside a wall cavity or an HVAC system, it is work for a remediation contractor.
On bleach, the guidance is worth repeating because the folk advice runs the other way. The EPA does not recommend using a chemical or biocide that kills organisms, chlorine bleach for example, as a routine practice during mold cleanup. It also warns never to mix a chlorine bleach solution with cleaners containing ammonia, because toxic fumes can be produced.
The summary
No thermostat setting makes a house safe. Keep the humidity under 60 percent, find the water feeding the room, and get wet material dry inside the 24 to 48 hour window.
DHL works to S520 for mold and to S500 for the water that caused it, and the two are usually the same job seen from different ends. The remediation method, including containment and post remediation verification, is on the Mold Remediation page, and the water side is on Water Damage Restoration.
This article is about Mold Remediation.